1. Field of the Invention
The present invention relates to a cathode catalyst for a fuel cell, and a membrane-electrode assembly for a fuel cell and fuel cell system including the same. More particularly, the present invention relates to a cathode catalyst having high activity for reduction of an oxidant and selectivity, and that is capable of improving performance of a membrane-electrode assembly for a fuel cell, and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas. Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte fuel cell has an advantage of a high energy density, but it also has problems in the need to carefully handle hydrogen gas and the requirement of accessory facilities, such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like, in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the polymer electrolyte fuel cell, but it has the advantages of easy handling of a fuel, being capable of operating at room temperature due to its low operation temperature, and no need for additional fuel reforming processors.
In the above fuel cell, the stack that generates electricity substantially includes several to scores of unit cells stacked in multi-layers, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (also referred to as a fuel electrode or an oxidation electrode) and a cathode (also referred to as an air electrode or a reduction electrode) attached to each other with an electrolyte membrane between them.
A fuel cell is a power generation system for generating electrical energy through oxidation of a fuel and reduction of an oxidant. The oxidation of a fuel occurs at an anode, while the reduction of an oxidant occurs at a cathode.
Both of the anode and the cathode include a catalyst layer that includes a catalyst to increase the oxidation of a fuel and the reduction of an oxidant. The catalyst for the anode catalyst layer representatively includes platinum-ruthenium, while that for the cathode catalyst layer may include platinum.
However, the platinum as a cathode catalyst has a problem of low reduction of an oxidant. It can also be depolarized by a fuel that crosses over toward a cathode through an electrolyte membrane, thereby becoming non-activated in a direct oxidation fuel cell. Therefore, what is needed is an improved catalyst for the cathode of the fuel cell that can be substituted for the platinum.